1. Field of the Invention
The present invention relates to an optical diffraction device, and more particularly to a binary-type optical diffraction device. The invention also relates to an optical system including a binary-type optical diffraction device and to an optical apparatus such as an exposure apparatus, including such an optical system.
2. Description of the Related Art
A binary device is regarded in the art as an important possible technique to realize a high-accuracy optical diffraction device. The binary device is a technique to produce an optical diffraction device having a step structure in cross section such as that shown in FIG. 1B, which is an approximation of the blazed cross-sectional structure of an optical diffraction device such as that shown in FIG. 1A. The optical device shown in FIGS. 1A and 1B is a diffraction-type Fresnel lens, which is also called a kinoform. As shown in FIGS. 1A and 1B, a diffraction grating having a fine structure is formed on a transparent substrate 100 or 101. If the structure of the diffraction device is approximated by a step structure, the diffraction device can be easily produced with high precision using the semiconductor process technology which is widely used to produce an LSI or the like.
FIGS. 2A-2H illustrate the process of producing a 4-step optical diffraction device (binary device) using the semiconductor process technology. In FIGS. 2A-2H, reference numeral 110 denotes a transparent substrate on which a diffraction grating is formed, 111 a resist coated on the substrate 110, and 112 a mask used to form a grating pattern. In the first processing step shown in FIG. 2A, the resist 111 is exposed via the mask 112 to exposure light 113 so as to form a latent image corresponding to the mask pattern. Then in the processing step shown in FIG. 2B, the resist is developed so that the portions exposed to light (latent image) are removed (herein the resist is assumed to be of the positive type). In the processing step shown in FIG. 2C, the substrate 110 is etched to a predetermined depth by means of the reactive ion etching technique. Then, the remaining resist is removed. At this processing stage, a 2-step structure is obtained as shown in FIG. 2D. In the following processing step shown in FIG. 2E, another resist 114 is coated on the surface of the substrate, and the resist 114 is exposed via a mask 114 having a grating pattern with a pitch half that of the pattern formed on the mask 112. In the processing steps shown in FIGS. 2F and 2G, the resist 114 is developed so that exposed portions of the resist are removed as in the previous development process, and the substrate 110 is etched using the remaining resist as a mask. After that, the remaining resist is removed. Thus, a 4-step structure is obtained as shown in FIG. 2H. When it is desired that the number of steps in the structure be increased, the above processing steps are repeated using a mask having a pattern with a pitch half that of the pattern formed on the mask 115. With this technique, although the number of steps in the structure is limited to 2.sup.n (n: integer), it is possible to form a desired number of steps by properly selecting the number of masks and the line width of the mask pattern.
In the above technique, a desired step structure is formed by etching a substrate. Instead, it is also known in the art to form a step structure by performing a process repeatedly to deposit a film having. a thickness equal to one step on a substrate in such a manner that the film is formed at predetermined locations. In the case where the diffraction grating structure is approximated by a step structure, although it is impossible to achieve 100% efficiency, high efficiency such as 95% for an 8-step structure and 99% for a 16-step structure can be achieved, which is good enough for practical applications.
FIG. 3 illustrates, in an enlarged fashion, a part of an optical diffraction device. With reference to this figure, the step structure will be discussed in further detail below. In this structure, it is assumed that the refractive index of the substrate 120 is n.sub.s and the refractive index of the medium 121 on which light is incident is n.sub.i. The broken line 122 represents the ideal structure of the device while the solid line 123 represents the structure which approximates the ideal structure by steps. In order for the incident light to encounter an abrupt phase change of 2.pi. at each boundary (denoted by B in FIG. 3) between adjacent units of the periodic structure, the height D of the ideal structure 122 should be EQU D=.lambda./(n.sub.s -n.sub.i)
where .lambda. is the wavelength of the light. If each step has a height h, and the number of steps is L (six steps are shown in FIG. 3 for convenience of explanation), the height E of the step-approximated structure 123 becomes E=(L-1)h. There is a difference, as denoted by .alpha. in FIG. 3, between the heights D and E, wherein .alpha. satisfies the following equation: EQU (L-1)h+.alpha.=.lambda./(n.sub.s -n.sub.i) (1)
Usually the structure is designed so that .alpha.=h. In this case, it is required to meet the following condition: EQU h=D/L and E=D.multidot.(L-1)/L
Thus, the height of each step is given by the above equation. However, the height of each step may also be determined in another way. In the above method, when the number L of steps is determined, the height h of each step is automatically determined, and it is impossible to modify h to optimize the characteristics of the device. From this point of view, it is rather desirable that .alpha. be allowed to have a value within the range 0&lt;.alpha.&lt;h so that h may be set to an arbitrary desired value. In this case, if .alpha. is represented as .alpha.=k.multidot.h (0&lt;k&lt;1), then the following equation should be met when the height h of each step and the number L of steps are determined. EQU (L-1+k)h=.lambda./(n.sub.s -n.sub.i) (2)
where k may have an arbitrary value within the range 0&lt;k .ltoreq.1.
The surface of an optical device is generally covered with an antireflection film for suppressing the reflection of light at the surface. In the case of a dioptric lens, since the surface is smooth, it is easy to form an antireflection film on the surface. As for binary devices, a technique of forming an antireflection film on the device surface is disclosed in "Anti reflection-coated diffractive optical devices fabricated by thin-film deposition", E. Pawlowski and B. Kuhlow, Opt. Eng. 33(11), 3537-3546 (1994). In this technique, as shown in FIG. 4, a material 131 for forming an antireflection film is deposited from above at right angle onto the surface of a substrate 130 by means of ion beam sputtering thereby forming a thin film 132 on the step-structured surface of the substrate 130.
FIG. 5 illustrates a multilayer antireflection film formed on the fine step-structured surface using the sputtering technique. As can been seen from FIG. 5, the antireflection film 143 formed on substrate 141 using the conventional technique has uniformity in thickness due to the steps, which causes a reduction in antireflection effect.
For a similar reason, a reduction in effect occurs when a reflection enhancement film is formed on a reflection type optical diffraction device.